building technology 3 - ohms law

8/12/2019 Building Technology 3 - ohms law

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Building Technology 3Assignment No. 1

Ohms Law

GULAPA, CARLO P.

AR1232


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Ohm's Law

Ohm's Lawdeals with the relationship between voltage and current in an ideal conductor.This

relationship states that:

The potenti al di ff erence (voltage) across an ideal conductor is proportional to the curr entthrough it.

The constant of proportionality is called the "resistance", R.

Ohm's Law is given by:

V = I R

where V is the potential difference between two points which include a resistanceR. I is thecurrent flowing through the resistance

Incircuit analysis,three equivalent expressions of Ohm's law are used interchangeably:

Each equation is quoted by some sources as the defining relationship of Ohm's law, or all three

are quoted, or derived from a proportional form, or even just the two that do not correspond to

Ohm's original statement may sometimes be given.

The interchangeability of the equation may be represented by a triangle, where V

(voltage)is placed on the top section, the I (current)is placed to the left section,

and the R (resistance)is placed to the right. The line that divides the left and

right sections indicate multiplication, and the divider between the top and

bottom sections indicates division (hence the division bar).

Brief History:In January 1781, before Georg Ohm's work, Henry Cavendish experimented with Leyden

jars and glass tubes of varying diameter and length filled with salt solution. He measured the

current by noting how strong a shock he felt as he completed the circuit with his body.Cavendish wrote that the "velocity" (current) varied directly as the "degree of electrification"(voltage). He did not communicate his results to other scientists at the time, and his results were

unknown until Maxwell published them in 1879.

Ohm did his work on resistance in the years 1825 and 1826, and published his results in 1827

as the book Die galvanische Kette, mathematisch bearbeitet ("The galvanic circuit investigated

mathematically"). He drew considerable inspiration from Fourier's work on heat conduction in
http://en.wikipedia.org/wiki/Circuit_analysishttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Circuit_analysis


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the theoretical explanation of his work. For experiments, he initially used voltaic piles, but later

used a thermocouple as this provided a more stable voltage source in terms of internal resistance

and constant potential difference. He used a galvanometer to measure current, and knew that thevoltage between the thermocouple terminals was proportional to the junction temperature. He

then added test wires of varying length, diameter, and material to complete the circuit. Ohm

determined his law of proportionality and published his results.

Understanding Ohms Law1. Material that obeys Ohm's Law is called "ohmic" or "linear"because the potential

difference across it varies linearly with the current.

2. Ohm's Law can be used to solve simple circuits. A complete circuit is one which is aclosed loop. It contains at least one source of voltage (thus providing an increaseofpotential energy), and at least one potential drop i.e., a place where potential energy

decreases. The sum of the voltages around a complete circuit is zero.

3. An increase of potential energy in a circuit causes a charge to move from a lower to ahigher potential (ie. voltage). Note the difference betweenpotential energy and potential.

Because of the electrostatic force, which tries to move a positive charge from a higher toa lower potential, there must be another 'force' to move charge from a lower potential to a

higher inside the battery. This so-called force is called the electromotive force, or emf.

The SI unit for the emf is a volt (and thus this is not really a force, despite its name). We

will use a script E, the symbol , to represent the emf.

4. A decrease of potential energy can occur by various means. For example, heat lost in acircuit due to some electrical resistance could be one source of energy drop.

Because energy is conserved, the potential difference across an emf must be equal to thepotential difference across the rest of the circuit. That is, Ohm's Law will be satisfied:

= I R
http://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.potE.htmlhttp://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.potE.htmlhttp://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.potE.htmlhttp://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.potE.html


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Sources:

http://en.wikipedia.org/wiki/Ohm's_lawhttp://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.intro.html

building technology 3 - ohms law

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